Magnetic quadrupole focusing system



April 19.59 H. F. KAISER EI'ALY v 2,883,569

MAGNETIC QUADRUPOLE FOCUS ING SYSTEM Filed Jan. 24, 1956 2 Sheets-Sheet 1 INVENTOIU HERMAN F KAISER WESLEY T. MAYES WILLIAM J. WILLIS I BY ' ATTORNEYS April 21, 1959 H. F. KAISER ETAL 2,883,569

MAGNETIC QUADRUPOLE FOCUSING SYSTEM I Filed Jan 24,1956 v 2 Sheets-Sheet 2 INVENTOPJ RMAN F. KAISER SLEY T. MAY ES WILLIAM J. WILLIS MMQQQMI ATTOR NE Y5 United States Patent 2,883,569 MAGNETIC QUADRUPOLE FOCUSING SYSTEM Herman F. Kaiser and Wesley T. Mayes, Washington, D.C., and William J. Willis, New Haven, Conn.

Application January 24, 1956, Serial No. 561,165 7 Claims. (Cl. 313-84) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates in general to particle focusing or deflecting systems and more particularly to a focusing system of the magnetic quadrupole type using four poles to set up a transverse magnetic focusing field for shaping a beam of charged particles.

An object of the invention is the provision of a quadrupole system utilizing permanent magnets with elimination of the necessity for the use of energizing windings with their source of current and controls therefor without sacrifice of fine adjustability of strength and form of the focusing field.

Another object is the provision of means for supplying the four pole pieces of a quadrupole transverse focusing field system with magnetic flux from permanent magnets, individually controlled for each pole piece to effect variation in strength and pattern of the focusing magnetic field.

Another object is the provision of a focusing system small in bulk, light in weight and stable in adjustment.

Various other objects and advantages of the invention will become apparent from a perusal of the following specification and the drawings accompanying the same.

In the drawings:

Fig, 1 is a diagrammatic showing of a transverse magnetic quadrupole focusing field formed by a quadrupole field system and its effect upon a stream or beam of charged particles passing through the field.

Fig. 2 is a diagram showing the cross-sectional outline of a beam of charged particles after passing through a second similar focusing field with reversed polarities.

Fig. 3 is a front elevation of an embodiment of the present invention.

Fig. 4 is a side view of Fig. 3.

Fig. 5 is an enlarged fragmentary section on the line 55 of Fig. 4.

Fig. 6 is a section on the line 6-6 of Fig. 5.

Referring to the drawings in detail and first to Fig. 1, here is shown diagrammatically the four pole pieces of a quadrupole transverse magnetic focusing system and the magnetic field pattern produced thereby. While theoretically an ideal contour for the pole pieces would be a hyperbolic curve it has been found in practice that semicylindrical pole pieces as here shown at 10, 11, 12 and 13 provide sufi'icient approximation to a hyperbolic surface to establish a satisfactory field pattern as shown. Here it will be noted that the lines of force between adjacent pole pieces of opposite polarity loop inwardly toward the center or axis of the system the lines of force neutralizing each other at or near the axis and increasing in strength away from the axis as indicated in the diagram by the increase in density or compactness of lines of force away from the axis or center of the array. An envelope 14 of glass or other suitable non-conducting, non-magnetic material for housing, in a vacuum or other 2,883,569 Patented Apr. 21, 1959 suitable environment, the beam of charged particles to be focused, is positioned axially within the circular array of pole pieces 10-13. Due to the law governing the movement of a charged particle through a magnetic field, a beam of general circular cross-section, as indicated in dotted lines at 15, of negative particles such as electrons, passing along the axis of the transverse focusing field in a direction away from the observer will be focused or compressed horizontally, and defocused or expanded vertically to form a ribbon shaped beam of vertically elongated cross-sectional contour as indicated at 16. This, it will be understood, results from the fact that the lines of force to the right of the axis of the beam are directed upwardly and increase in density away from the beam in a direction to the right while the lines of force to the left of the beam are directed downwardly and increase in density away from the beam in a direction to the left, so that negatively charged particles to the right of the axis are forced toward the center while those to the left of the axis are also forced toward the center. On the other hand, particles above the center of the beam passing through the lines of force directed from left to right between the top pole pieces 13 and 10 are deflected upwardly while those passing along the lower portion of the beam below the axis traversing the field between the lower pole pieces 11 and 12 are deflected downwardly. To reform the ribbon beam 16 into a compact rod-like beam suitable for use in a variety of electronic and nuclear accelerating devices, it is simply necessary to pass the ribbon beam through a second focusing field like that of Fig. 1 but of opposite polarities which now acting in opposite directions compresses the ribbon beam vertically with but slight horizontal expansion, to bring it to a rodlike form of cross-sectional contour of somewhat cushion shape as indicated in Fig. 2. To enable the establishment and control of a transverse magnetic focusing field as above described, with elimination of conventional energizing windings and their accessories such as a current source and current control means, with their attendant weight and bulk, the present invention provides a structure as exemplified in Figs. 3 to 6. Here the pole pieces 10-13 are mounted in circular array symmetrically about an axis 18 and held in fixed position in a yoke or frame 19 of non-magnetic material such as aluminum or brass, and with stem sections 20 (Figs. 5 and 6) extending in the form of short cylindrical sections through to the external face of the yoke. A permanent magnet bar 21 for each pole piece is positioned to supply magnetic flux to its associated pole piece and a magnetic circuit return element 22 in the form of a closed ring. The permanent magnet rods 21 may sit directly on the pole stems 20 held there by magnetic attraction, or may be slightly spaced by an interposed shim of non-magnetic material for a purpose to be hereinafter described. A set of telescoping inner and outer sleeves 23 and 24, respectively, of low reluctance material provide magnetic connection between the outer end of each magnet bar 21 and the return ring 22. A back plate 25 of non-magnetic material such as aluminum or brass, mounts the return ring 22 and yoke 19 in fixed relation, the back plate being provided with a central opening 30 to admitpassage of the tubular envelope 14. The yoke is secured to the back plate 25 by means of suitable screw bolts 26 (Fig. 6) threaded into the threaded openings 27 at the corners of the yoke 19. The return ring 22 is also secured to the back plate 25 by suitable screw bolts not shown. A cover element 28 which may be of transparent non-magnetic material such as Lucite is secured in place by suitable screw bolts 29 which take into the threaded corner openings 27 in the yoke, the cover being provided with a central opening 31 concentric with the opening 30 in the back plate to permit passage of the envelope 14 through the assembly co- 3 axially with the axis 18. In use, with the permanent magnetic bars 21 positioned to magnetize adjacent pole pieces at opposite polarities as indicated in Figs. 1 and 3, the magnetic flux paths are completed through the return ring 22 to which the outer ends of the permanent magnet bars 21 are magnetically connected through the sleeve elements 23, 24. This completes the four magnetic flux paths to form the quadrupole transverse magnetic focusing field as shown in Fig. 1. One such circuit path for example that of the magnetic flux supplied by the permanent magnet bar for yoke pieces 11 and 10, may be traced from the north pole N of the permanent magnet bar 21 for pole piece 11, polepiece 11, across the field gap to pole piece 10, south pole S of the permanent magnet bar for pole piece 10 north pole N of that bar, and back to the south pole S of the permanent magnet bar for pole piece 11 via the return ring 22 and the interposed low reluctance sleeves.

With the focusing field of Fig. 1 thus established, any lack of symmetry in the focusing field or any undue strength or weakness of the field as a Whole may be corrected by suitable axial adjustment of the inner sleeve elements 23 in the outer sleeve elements 24 to effect a lengthening or shortening of the extensible shunting sleeve formed by the sleeve elements 23-24. This adjustment of the sleeve elements 23--24 alters the effective length of the permanent magnet bars by shunting a portion of the magnetic flux from end-to-end of the magnet independently of the return'ring 22,

An alternative method of varying the amount of flux supplied by the permanent magnet bars to the pole pieces may be by varying the reluctance of the magnetic circuit by interposing a gap between the end of the permanent magnet bar 21 and its pole piece which may be accomplished by any known or other suitable means such as the use of a shim 32 (Fig. 3). Use of the variable gap in the magnetic circuit gives a coarser adjustment than does the use of the variable magnetic shunt. However, where desired, both adjustments may be used together, the variable gap for coarse adjustment and the variable magnetic shunt for fine adjustment. Rotation of the inner sleeve 23 for adjustment of the effective length of the telescoping sleeve assembly may be effected readily from the outside by a suitable tubular spanner wrench slidable over the outer end of the permanent magnet bar and provided with diametrically spaced lugs to be engaged in the diametrically spaced radial grooves 33 (Fig. in the end of the inner sleeve element 23.

Where it is desired to control strength of magnetic field by varying an air gap in the magnetic circuit, the inner sleeve element 23 may be locked to the permanent magnet bar by means of the countersunk set screw 34 provided for the purpose. This enables the production of a variable air gap between the inner end of the magnet rod and the stem of the pole piece, variation of the air gap being effected by simply rotating the inner sleeve 23.

While but one specific embodiment of the invention has been herein described for the sake of disclosure, it is to be understood that the invention is not limited to such specific embodiment, but contemplates all such modifications and variants thereof as fall fairly within the scope of the appended claims.

What is claimed is:

1. In a magnetic lens system having an array of four pole pieces arranged symmetrically about a given axis along which a beam of charged particles is to be passed, and a magnetic return yoke ring surrounding the array of pole pieces, the combination of a permanent bar magnet for each pole piece for supplying magnetic flux each to its pole piece and the return yoke, and means for controlling the amount of flux supplied by each magnet to its pole piece.

2. In a magnetic lens system having an array of four pole pieces arranged symmetrically about a given axis along which a beam of charged particles is to be passed,

and a magnetic return yoke ring surrounding the array of pole pieces, the combination of a permanent bar magnet for each pole piece for supplying magnetic flux each to its pole piece and the return yoke, and means for controlling the amount of flux supplied by each magnet to its pole piece, each said control means comprising an extensible shunting sleeve of low reluctance material surrounding the magnet bar and in contact at its outer end with said return yoke.

33. In a magnetic lens system having a circular array of pairs of pole pieces arranged symmetrically about the axis of a beam of charged particles to be focused and a yoke ring of low reluctance material surrounding the array of pole pieces, the combination of a permanent magnet bar for each pole piece having one end presented to its pole piece for directing magnetic lines of force thereto and the other end magnetically coupled with the yoke ring through a telescoping sleeve element of low reluctance material, whereby the sleeve may be telescopically varied in length to variably shunt the magnet bar.

4. In a magnetic lens system having a circular array of pairs of pole pieces arranged symmetrically about the axis of a beam of charged particles to be focused and a yoke ring of low reluctance material surrounding the array of pole pieces, the combination of a permanent magnet bar for each pole piece having one end presented to its pole piece for directing magnetic lines of force thereto and the other end magnetically coupled to the yoke through a telescoping sleeve of low reluctance material having one end fixed in said yoke and the other end extended along the magnet bar from the yoke toward the pole piece, whereby telescopic extension and contraction of the sleeve will effect variable shunting of the magnet bar through the sleeve.

5. In a magnetic lens system having a circular array of pairs of pole pieces arranged symmetrically about the axis of a beam of charged particles to be focused and a yoke ring of low reluctance material surrounding the array of pole pieces, the combination of a permanent magnet bar for each pole piece having one end presented to its pole piece for directing magnetic lines of force thereto and the other end magnetically coupled with the yoke ring through a telescoping sleeve element of low reluctance material, whereby the sleeve may be telescopically varied in length to variably shunt the magnet bar to effect gradual adjustment of the amount of magnetic flux supplied by the magnet bar to its pole piece and the yoke ring, and means for holding said one end of the magnet bar in given spaced relation to its pole piece.

6. In a magnetic lens system having a circular array of pairs of pole pieces arranged symmetrically about the axis of a beam of charged particles to be focused and a yoke ring of low reluctance material surrounding the array of pole pieces, the combination of a permanent magnet bar for each pole piece having one end presented to its pole piece for directing magnet lines of force thereto and the other end magnetically coupled with the yoke ring through a telescoping sleeve element of low reluctance material, whereby the sleeve may be telescopically varied in length to variably shunt the magnet bar and effect gradual adjustment of the amount of magnetic lines of force supplied by the magnet bar to its pole piece and the yoke ring, and means for variably spacing said one end of the magnet bar from its pole piece to efiect a coarse adjustment of the magnetic coupling between the bar and its pole pieces.

7. In a magnetic lens system having a circular array of pairs of pole pieces arranged symmetrically about the axis of a beam of charged particles to be focused and a yoke ring of low reluctance material surrounding the array of pole pieces, the combination of a permanent magnet bar for each pole piece having one end presented to its pole piece for directing a magnetic flux through its pole piece and the other end magnetically coupled to the yoke through a pair of overlapping concentric sleeves of 2,ss3,5ee

low reluctance material, the outer of said sleeves being closely fitted into an opening in the yoke and the inner sleeve in free sliding engagement with the magnet bar and in threaded engagement with the outer sleeve whereby rotation of the inner sleeve Will eifect a variation in length of the portion of the magnet bar covered by the sleeve assembly.

References Cited in the file of this patent UNITED STATES PATENTS Hillier Dec. 7, 1948 Webb June 27, 1950 Rudenberg Aug. 29, 1950 Flory Ian. 31, 1956 

